Such a method is disclosed in U.S. Pat. No. 4,865,085 (corresponding to EP-0 199 059 BI). In this method, the sensor device operates with a receiver which monitors the axial movement of yarn windings on a stationary storing drum, and a second receiver monitors the quality of the light transmission. An output signal of the second receiver is compared with a threshold value in order to provide an additional useful signal which serves to increase the light intensity for both receivers when the light transmission has deteriorated. Also, an alarm signal for an operator can be generated indicating the necessity for cleaning of the light transmission path by removing contaminants which disturb or block the light transmission.
In a method disclosed in U.S. Pat. No. 4,963,757, a light source feeds two receivers, one of which scans a yarn and the other scans only the light transmission quality in order to maintain the relation between the output signals of both receivers substantially constant, and to compensate for a deterioration of the light transmission quality.
According to a method disclosed in U.S. Pat. No. 3,907,440, phase-offset light pulses for one receiver are generated by means of two pulsed light sources, and a yarn is scanned only with the light pulses of one of the light sources. The output signals originating from the light pulses not used for yarn scanning are compared with a nominal signal value in order to maintain a predetermined relation between both signals and to compensate for disturbing influences.
GB-A-22 27 092 discloses an optoelectronic sensor which consists of a light source and a receiver. The sensor is checked in a bank-note receiving and discharging device as to the instantaneous scanning characteristics before the sensor takes part in the checking of a bank note. In a test routine, a state, as may later be found in testing a bank note, is simulated by darkening the light source at the control side, as compared with the normal light intensity of the light source. The level of resulting output signals of the receiver is compared with a threshold value level calculated by the control device from those output signals of the receiver that are obtained without and with the darkening action. If the level of the darkened output signal is below the threshold value level, an alarm will be initiated.
According to another method known from WO95/16628 and used for controlling the drive motor of a yarn feeding device for a knitting machine, the yarn feeding device includes a rotatably driveable storing drum defining a storing surface and a stationary sensor device. Circumferentially offset surface areas of the storing surface are simultaneously optoelectronically scanned in the scanning zone by means of a plurality of sensors. In case where yarn is present in the scanning zone, the sensors simultaneously output identical output signals. In contrast, when yarn is absent from the scanning zone, the sensors simultaneously generate different output signals. By discrimination between the output signals, control signals are derived, and the drive motor is driven as long as the scanning zone is free from yarn, until yarn reaches the scanning zone again. When replenishing the yarn store, i.e., in the driven state of the drive motor, a speed signal for the control circuit is derived from the output signal of a sensor. A predetermined quality of the light transmission is required for the operation of the sensor device. Contaminants and lint, which unavoidably occur when processing yarns, deteriorate the quality of the light transmission with increasing duration of operation. The sensor device then fails and the storing surface becomes empty, and this might lead to a defect in the product produced in the textile machine which is being supplied with yarn by the yarn feeding device. Therefore, it is customary operator cleans the light transmission path within periods based upon experience, e.g. by pressurized air or by sweeping. However, the cleaning steps are then carried out more often than necessary, or a disturbance occurs due to a lack of care of the operator.
It is the object of the present invention to provide a method of the kind as disclosed above, as well as a yarn feeding device which enable reliable detection of deteriorated scanning conditions which just barely allow correct operation of the sensor device in a structurally simple way and with a simple circuitry technique. The invention also enables elimination of these less than optimal scanning conditions so as to avoid defects in the product produced by the textile machine supplied with yarn by the yarn feeding device.
In the method according to the invention, an object-output signal generated for control purposes is also used to check the quality of the scanning conditions, e.g. the quality of the light transmission, by means of surface areas of the storing surface and/or a yarn which serve as the object. This does not require any appreciable additional components in the sensor device, or at the storing surface. The scanning conditions (or the light transmission quality) decisive for the function of the sensor device are examined in the scanning zone, i.e. at the exact location where they are decisive for the function of the sensor device for controlling the drive motor, and not at a location which is distant from the scanning zone. A deterioration of the scanning conditions will change the signal level of the output signal, and also the signal level of the test signal which is compared with a threshold value. The threshold value is set so as to correspond to a just barely acceptable deterioration of the scanning conditions. When the test signal finally falls below the threshold value the alarm signal is activated. By means of the alarm signal, an operator becomes alarmed just in time, i.e. neither too early nor too late, to clean the operating area of the sensor device, i.e. for example the light transmitting path. However, the alarm signal can also be used to automatically activate a cleaning device for the sensor device, which cleaning device automatically carries out a cleaning step, e.g. by blowing away or sweeping away contaminants.
In the yarn feeding device, an examination of the scanning conditions is made exactly at the location at which the object is scanned, i.e. at a location where the quality of the scanning condition is of decisive importance for correct functioning of the sensor device. Since the object-output signal itself is used as a basis for the test signal, no additional sensor components or auxiliary means are needed at the storing surface. Components already used for scanning the object are also used for the test routine, and as a result, the scanning condition is checked during operation periods only, and if scanning conditions deteriorate during an operation period, an alarm signal is generated which alerts the operating personnel to remove the disturbance is generated, during which operation periods a deterioration of the scanning condition might disturb the operation of the sensor device. Further, the examination is not carried out permanently, i.e. it is not carried out during unimportant time periods in which the scanning condition is of no influence on the operation of the sensor device. The structural features provided are advantageous with yarn feeding devices having a storing surface driven by the drive motor (rotatably driven storing body) as well as with yarn feeding devices having a stationary storing surface during operation (stationary storing drum and rotatably driven winding element), in order to reliably determine when a disturbance has to be eliminated.
The method according to the invention also includes monitoring the test signal and the object-output signal, and a simple logical evaluation of the occurrence or the non-occurrence of both signals is carried out in order to generate the alarm signal at a correct point in time and on the basis of a correctly determined scanning condition.
Further, the test signal, as well as a speed signal for control purposes of the drive motor, are generated from the object-output signal. An examination of the scanning conditions is only carried out in the event that the drive motor must be driven when there is danger of emptying the storing surface. Although the alarm signal is generated when the test signal fails to appear, the speed signal still appears for unobstructed use.
According to another aspect of the invention, the output signal and the test signal are both compared with separate threshold values. The higher threshold value represents a just barely acceptable deterioration of the scanning conditions. The output signal and the test signal not only occur synchronously with one another, but are also identical in their signal levels which is decisive for comparison with the respective threshold value. Since the threshold value for the test signal is higher, the test signal fails to appear as soon as the just barely acceptable deterioration has occurred. The output signal is still present and can be used in the predetermined way for control purposes. In the absence of the test signal, however, the alarm signal is generated. The lower threshold value can suitably be set to correspond to a worse deterioration of the scanning conditions (as compared to the threshold value for the test signal) at which a correct operation of the sensor device is no more assured. In the event that the operator has not reacted to the alarm signal, the yarn feeding device, and appropriately also the textile machine supplied with yarn by the yarn feeding device, can be switched off when the output or speed signal also fails to appear, in order to avoid emptying of the storing surface.
Alternatively, both signals can be compared with the same threshold value. Prior to this comparison, the signal level of the test signal is changed so that upon the comparison of its signal level with the alarm-threshold value, precise information is gained indicating the need of an alarm signal.
The method is particularly useful with opto-electronical and contactless scanning in a yarn feeding device comprising an opto-electronic sensor device predictable relation between the signal level and the quality of the light transmission.
However, the application of this method, and the structural features for carrying out the method are not limited to optoelectronical scanning, and it is possible to use an output signal generated for a predetermined control purpose also for a testing routine with other contactless scanning modes (sound, induction, etc) and even with contact yarn scanning. It is, however, important that the output signal used for the test routine originates from the scanning of the object in the scanning zone and shows an easily evaluatable signal level which changes with a deterioration of the scanning conditions, e.g. due to dirt or dust deposits. The principle of the invention is also useful for yarn feeding devices having a stationary storing surface for the yarn. In this situation, the output signal need not necessarily be in the form of a signal chain, even though this might be advantageous in some cases.
With the yarn feeding device according to the invention, the object-output signal is used for the testing routine. The object-output signal represents the rotational speed of the drum and occurs exclusively when the drum is driven due to absence of yarn in the scanning zone. By means of the test signal which is formed from the object-output signal, the alarm signal can be generated simply and reliably, and exactly at a point in time when the scanning conditions have deteriorated accordingly. One particularly useful feature of the invention is that the operation of the sensor device is only checked when the drive motor is driven for replenishing the yarn store. In such a case, there is the risk of emptying the storing surface, because the boundary of the yarn store trails behind the scanning zone due to consumption. If the drive motor is not driven, no test routine is carried out. This is then uncritical, because there is a sufficiently big yarn store already on the storing surface, which yarn store extends into the scanning zone. Elimination of the disturbance or cleaning is carried out expediently when the drive motor is at a standstill, so that the yarn feeding device does not have to be switched off, and the production process of the textile machine, which is supplied with yarn by the yarn feeding device, does not have to be interrupted.
According to one embodiment of the yarn feeding device, despite the absence of the test signal, the still occurring output signal is used as a speed signal for control purposes, and the alarm signal is generated separately. It is useful to use the microprocessor of the control device of the yarn feeding device (which microprocessor is typically provided in any event) as a combining or monitoring means for the above purpose, because the microprocessor usually has sufficient capacity for this additional program routine and thus only requires a software adaptation.
Further, in accordance with another embodiment, the microprocessor switches off the yarn feeding device, and, expediently, also the textile machine supplied with yarn from the yarn feeding device, via a switch or switch-off member, namely as soon as the speed signal compared with the threshold value also fails to appear. The invention thus incorporates a dual-safety feature in the event that the operator fails to respond to the alarm signal and eliminate the disturbance.
In another embodiment, a voltage divider produces the same signal level for the output signal and the test signal, and two comparators compare the two signal levels with two different threshold values. As a consequence, the speed signal which may possibly be needed for control purposes still occurs, even when the scanning conditions have deteriorated to a just barely acceptable degree, although the test signal has failed to appear and the alarm signal is generated.
In contrast, in an alternative embodiment, the signal level for the test signal is changed in relation to the signal level of the output signal already in the voltage divider. The speed signal which may possibly be needed for control purposes can still be derived from the output signal, while with a just barely acceptable deterioration of the scanning conditions, the test signal fails to appear and the alarm signal is generated.
In another embodiment, a very reliable, preferably opto-electronical, yarn scanning with a precise control of the drive motor is achieved by providing the plurality of single sensors, with only the output signal of one of the single sensors being used for the test routine.